Relates to an arrangement for measuring angular velocity. An arrangement in accordance with the invention may be used in a motor vehicle to detect a roll-over accident to control the deployment of a safety device such as an air-bag.
Vibrating angular velocity meters/gyros of the tuning fork type, or which use other vibrating members, have heretofore been constructed and tested both in metal and quartz and also in silicon. Both types may be constructed with an open loop and with a closed feed-back loop. The advantage of quartz and silicon based gyros over the prior proposed metal-fork gyro (Barnaby et. al., Aeronautical engineering review, p. 31, November 1953) is that they can be miniaturised and manufactured relatively cheaply in large production runs by batch manufacture.
Unlike rotating gyros in which the Coriolis force develops a constant torque during a turn, turning of a vibrating gyro results in an oscillating torque in which the amplitude is proportional to the turning speed and the phase indicates the turning direction. As a result, relatively extensive electronic circuitry is required for excitation and detection and interpretation of the gyro signal from a vibrating gyro as compared with a conventional rotating gyro.
Since silicon is a well-developed electronic material, there is considerable advantage in using a silicon-based gyro integrated together with associated excitation and sensor electronics. However, it has not been possible to produce and excite a traditional tuning fork in silicon easily.
The advantage of a tuning fork construction is that it can be made to swing in a dynamically balanced mode which makes the gyro insensitive to vibration and which gives a high Q-factor since little vibration energy is yielded to the surroundings. Even if it is possible, in principle, using moderm plasma etching techniques, to etch out, in silicon, a tuning fork (or any other horizontally swinging structure (Satchell, UK patent application, GB2198231A)), there still remains the excitation problem.
In silicon it is not possible to utilise the piezo-electric effect directly for excitation as in the case of quartz, since silicon is not a piezo-electric material. Of course a piezo-electric layer can be deposited on silicon, but if the structure has been etched out vertically down from the substrate surface, this means that a piezo-resistive layer of constant thickness has to be deposited on vertical walls, and this can be fairly problematic.
Instead, the hitherto most common solution to the excitation problem for silicon-based gyros has been to use electrostatic excitation at right angles to the substrate surface by making conductive plates on one or both sides of a bar or a swashplate (possibly with a weight thereon), or alternatively quite simply to xe2x80x9cgluexe2x80x9d practically the entire component on a piezo-electric plate.
Unfortunately, the micromechanical gyros previously proposed using electrostatic excitation at right angles to the substrate surface, which may be relatively easily produced, have suffered from a low gyroscopic scale factor of the order of magnitude of 0.01-0.2 (Boxhorn, U.S. Pat. No. 4,598,585). This can be compared with a tuning fork, which has a gyroscopic scale factor of 2 but which is much more difficult to produce, since it requires excitation parallel to the substrate surface, because the limbs must swing in the plane of the substrate in anti-phase with each other.
According to one aspect of this invention there is provided a sensor adapted to respond to a rotation, said sensor comprising a body formed from a substantially planar substrate, said body incorporating a beam, the beam having two opposed ends, each end being adapted to be fixed in position, the beam being provided with at least one inertia mass connected to the beam at a predetermined position, the body being associated with means for exciting a first oscillation of the inertia mass substantially about an axis perpendicular to the plane of the substrate, there being means for detecting a second oscillation of the mass, substantially around an axis coincident with the longitudinal axis of the beam, which second oscillation is caused by the coriolis force that arises when the body is subjected to rotation around an axis having at least a component in the said plane, but perpendicular to the said axis of the beam, the beam being configured so that the beam bends most easily in one predetermined direction, that direction making an acute angle with the plane, so that the direction is neither parallel with, nor perpendicular to the plane, so that the first oscillation may be initiated by an excitation force which is not parallel with said plane.
Preferably a sensor according to claim 1 wherein the beam is such that the first oscillation may be caused by an excitation force which is substantially perpendicular with said plane.
Conveniently the predetermined direction in which the beam bends most easily is at approximately 55xc2x0 to the plane of the substrate.
Advantageously the or each inertia mass comprises two elements, located respectively on either side of the beam, within the plane of the substrate, the two elements being interconnected by a connecting bar, the central portion of the connecting bar being unitary with a portion of the beam.
Preferably the means for exciting the first oscillation of the or each inertia mass comprise at least one capacitative plate located adjacent a selected portion of the first inertia mass, and means to apply selected potentials to the capacitative plate, the inertia mass being provided with a conductive portion, there being means to apply a predetermined potential to the conductive portion, the arrangement being such that the potentials applied to the capacitative plate causes part of said inertia mass to tend to be deflected out of the plane of the substrate.
Conveniently there is a plurality of said capacitative plates.
Advantageously the or each capacitative plate is on a substrate of glass or semi-insulating silicon.
Preferably the means for applying potentials to said capacitative plate or plates are adapted to supply signals which generate a xe2x80x9csee-sawxe2x80x9d motion of the inertia mass about the connecting bar, the effect of the configuration of the beam being such that this oscillation generates the said first oscillation about the axis perpendicular to the plane of the substrate.
Conveniently the means for detecting the second oscillation comprise at least one further capacitative plate located adjacent on selected portions of the first inertia mass, and means to measure the capacitance between said further capacitative plate and the said body to detect said second oscillation.
Advantageously there is a plurality of said further capacitative plates.
In one embodiment there is only a single inertia mass, that inertia mass being located substantially centrally of the beam, half-way between the means provided at the opposed ends of the beam for mounting the beam in position.
Preferably the inertia mass is symmetrical about the axis of the beam.
In an alternative embodiment there are two inertia masses, the inertia masses being symmetrically disposed on the beam, each inertia mass being located at a position between the center part of the beam and a respective end of the beam.
Conveniently the two inertia masses are identical and symmetrical about the axis of the beam.
Preferably each inertia mass is provided with means for exciting a first oscillation of the mass around an axis perpendicular to the plane of the substrate, the arrangement being such that the masses oscillate in anti-phase.
Advantageously the body is formed from a mono-crystalline substrate, such as silicon.
The sensor may be adapted to provide an output signal representative of angular velocity.
The invention also relates to a sensor as described above mounted on a motor vehicle to control deployment of a safety device.
According to another aspect of this invention there is provided a sensor adapted to respond to a rotation, said sensor comprising a body formed from a substantially planar substrate, said body incorporating a beam, the beam having two opposed ends, each end being adapted to be fixed in position, the beam being provided with inertia mass means connected to the beam, the body being associated with means for exciting a first oscillation of the inertia mass means, that first oscillation being an oscillation substantially within the plane of the substrate, there being means for detecting a second oscillation of the inertia mass means, substantially around an axis coincident with the longitudinal axis of the beam, which second oscillation is caused by the coriolis force which arises when the body is subjected to a rotation about an axis, having at least a component in the said plane, but perpendicular to the said axis of the beam, wherein the inertia mass means comprises at least two discrete inertia masses, located at different longitudinal positions on the beam, the said two masses being excited to oscillate in anti-phase. Conveniently the predetermined axis is within the plane of the substrate and the perpendicular axis is coincident with the longitudinal axis of the beam. Preferably there are two inertia masses, the two inertia masses are symmetrically disposed on the beam, each inertia mass being located at a position between the center part of the beam and a respective end of the beam.
Conveniently the two inertia masses are identical and symmetrical about the axis of the beam.
Advantageously each inertia mass comprises two elements, located respectively on either side of the beam, within the plane of the substrate, the two elements being interconnected by a connecting bar, the central portion of the connecting bar being unitary with a portion of the beam.
Preferably the first oscillation may be caused by an excitation force which is substantially perpendicular with said plane.
In a preferred embodiment the means for exciting the first oscillation of the inertia masses comprises a plurality of capacitative plates, at least one capacitative plate being located adjacent a selected portion of each inertia mass, and means to apply selected potentials to the capacitative plates, the inertia masses each being provided with a conductive portion, there being means to apply a predetermined potential to the conductive portion provided on each inertia mass, the arrangement being such that the potentials applied to the capacitative plates cause part of each inertia mass to tend to be deflected out of the plane of the substrate. Advantageously each capacitative plate is on a substrate of glass or semi-insulating silicon.
Preferably the means for applying potentials to said capacitative plates are adapted to supply signals which generate a xe2x80x9csee-sawxe2x80x9d motion of each inertia mass about the connecting bar, the beam being configured so that the beam bends most easily in one predetermined direction, that direction making an acute angle with the plane of the substrate, so that the direction is neither parallel with, nor perpendicular to the plane, so that this xe2x80x9csee-sawxe2x80x9d oscillation generates the said first oscillation of each inertia mass about an axis perpendicular to the plane of the substrate.
Conveniently the means for detecting the second oscillation comprise further capacitative plates, at least one further capacitative plate being located adjacent a selected portion of each inertia mass, there being means to measure a capacitance between each of said further capacitative plates, and the said body to detect said second oscillation.
Preferably the body is formed from a mono-crystalline substrate, such as silicon
The sensor may be adapted to provide an output signal representative of angular velocity.
The invention also relates to a sensor in accordance with the second aspect of the invention mounted on a motor vehicle to control deployment of a safety device.
The preferred embodiment of the present invention provides a vibrating gyro construction formed from a substrate which is adapted to be excited electrostatically unilaterally, with a gyroscopic scale factor greater than 0.2, and which is in the form of a dynamically balanced structure which is insensitive both to linear acceleration and to angular acceleration. This is achieved by suspending the gyroscopic mass or masses on a bar which is configured so that it bends most easily in one predetermined directionxe2x80x94herein termed the xe2x80x9csoftxe2x80x9d bending directionxe2x80x94which is not at right angles to or parallel to the substrate normal plane. As a result, the gyroscopic mass or masses can be made to move mainly in the plane of the substrate when an electrostatic excitation mainly at right angles to the substrate is applied with the correct frequency. In this way, the mass movement can be utilised so that a large gyroscopic torque is obtained out of the plane when the arrangement is subjected to a rotation in the plane at right angles to mass movement. Previous cardanically suspended constructions, such as that known from Swedish patent SE9500729-0 have the considerable disadvantage of a low gyroscopic scale factor, which makes the gyro vibration-sensitive, especially if it does not swing in a completely balanced modexe2x80x94something which was hitherto impossible with unilateral excitation and detection.
The preferred embodiment of the present invention extends the technology of vibrating gyro constructions formed from a semi-conductor substrate to allow integration with a three-axis accelerometer if the same production technique known from Swedish patent SE9203648-2 is utilised to embody a bar whose xe2x80x9csoftxe2x80x9d bending direction is not at right angles to or parallel to the substrate normal plane, and permits the production of vibrating gyros in the same substrate for the simultaneous measurement of rotation around two axes at right angles to one another. Depending upon the application requirements, the preferred embodiments of the invention can be integrated in a number of configurations in the same substrate which may also incorporate accelerometers. This permits simultaneous measurement of rotation around a plurality of axes, possibly in combination with acceleration measurement. Also, depending upon the accuracy requirements of the application, the arrangement can be embodied either with an open or closed feed-back loop. The use of well-developed silicon technology in the production process permits mass production at low cost with high accuracy and reliability.
One preferred embodiment of the arrangement according to the invention for measuring angular velocity comprises a bar whose soft bending direction is not at right angles to or parallel to the normal to the substrate with a centrally disposed inertia mass which can be etched out from the same material as the bar. One example of such a material is silicon, which can be doped to produce conductive elements where required.
Another preferred embodiment of the arrangement according to the invention for measuring angular velocity comprises a bar whose soft bending direction is not at right angles to or parallel to the normal to the substrate and two inertia masses disposed along the bar which can be etched out from the same material as the bar. An example of such a material is silicon, which can be doped to produce conductive elements where required.
In both of these arrangements the inertia masses are connected to the rest of the substrate by a flexible bar which has a bending direction which is not at right angles to or parallel to the normal to the substrate. This can be achieved in various ways, for example, by anisotropic etching from opposite surfaces of a semi-conductor substrate to create a bar angled to the normal plane in accordance with SE9203648-2, and by making this connect the inertia masses to the substrate. Alternatively, anisotropic etching which etches at an oblique angle to the normal plane, may be combined with dry etching, which etches at a right angle to the normal plane, to produce a bar of triangular cross-section. Alternatively, it is possible to etch away one corner of a bar of rectangular cross-section with the result that the preferred bending direction is changed so that the preferred bending direction is no longer at right angles to the long sides of the rectangular cross-section. The bar width and thickness can, for all the above examples, be dimensioned so that high bending resilience is achieved along the required axis.
Both configurations discussed above are suitable for an embodiment of the type with an open loop and for an embodiment of the type with a closed feed-back loop. Hybrid embodiments are also feasible, in which a system with a closed feed-back loop is produced with a long time constant so that the gyro reacts to abrupt changes while slow drifts are cancelled out by feedback. The greatest advantages of a hybrid construction are that the mechanical amplification (Q-factor) in the direction of detection, obtained at the resonant frequency, can be utilised and that all the static sources of error are automatically cancelled out while the long-term drift problem is eliminated. In a conventional system with a closed loop, the Q-factor remains at 1 as a result of the high feedback. The hybrid construction nevertheless has the disadvantage that it is possible to measure only changes in rotational velocity, and not constant rotational velocities, since the constant signal, in time, is cancelled out by the weak feedback. The sensor has a high-pass characteristic whose cut-off frequency is determined by the feedback time-constant.
An open-loop construction is also possible, but again requires some type of balancing of the structure either mechanically or more attractively by means of DC voltage. Since the electrostatic force has a non-linear dependency on the distance between the electrodes, as in the case of a plate capacitor, it is possible to introduce negative spring constants electrically by means of DC. This makes it possible to compensate for mechanical imbalance and electrical imbalance in the structure by applying different DC voltages for excitation, and to detection plates, and possibly also to extra balancing electrodes, in accordance with a predetermined appropriate pattern.
An arrangement in accordance with the invention may be produced in semi-conductor material by well-known semi-conductor production methods comprising, for example, photolithographic patterning, isotropic and anisotropic etching. This provides many advantages including close tolerance control, and the possibility of integrating all or some of the signal processing electronics in a single common substrate of relatively moderate thickness, while providing access to a technology which permits effective mass-production by batch manufacture. Patterned conductive surfaces can be placed on the surface of the bar and the inertia masses, for example by xe2x80x9cAnodic Bondingxe2x80x9d or xe2x80x9cSilicon Direct Bondingxe2x80x9d. xe2x80x9cAnodic Bondingxe2x80x9d allows quartz glass to be bonded to silicon, oxides, nitrides and metals at relative low temperatures (usually 300-400xc2x0 C.), by applying an electric field over the joint. xe2x80x9cSilicon Direct Bondingxe2x80x9d, which has been known since 1986 (Lasky, Applied Physics Letters Vol. 48, p. 78, 1986,) allows bonding, for example, of silicon to silicon, silicon to silicon dioxide and silicon dioxide to silicon dioxide. xe2x80x9cSilicon Direct Bondingxe2x80x9d and xe2x80x9cAnodic Bondingxe2x80x9d can also be utilised to attach mechanical stops to the bar so that it is not broken if it is subjected to greater forces than intended. If the bonding is carried out in vacuo, the techniques can be used simultaneously with a method of vacuum encapsulation of the arrangement.